Rinse and drain lentils. In a small saucepan combine lentils, water, and 1/4 teaspoon salt. Bring to boiling; reduce heat. Cover and simmer for 15 to 20 minutes or until the lentils are tender. Drain lentils; rinse with cold water. Drain again.

For dressing, in a small mixing bowl stir together yogurt, parsley, 1 tablespoon milk, mustard, sugar, 1/4 teaspoon salt, and pepper. Pour over lentil mixture. Toss to coat. Cover and chill for 2 to 24 hours.

To serve, cut one of the tomatoes in half crosswise; chop one of the tomato halves. Stir chopped tomato into lentil mixture. If necessary, stir 1 to 2 tablespoons milk into lentil mixture to moisten.

Line individual salad plates with lettuce. Cut the remaining tomato half and the whole tomato into slices. Then cut the tomato slices in half. Divide tomatoes among the lettuce-lined plates. Spoon lentil mixture over the tomatoes. Makes 4 main-dish servings.